The Mud Battery
The 1936 discovery of a pottery jar at Khujut Rubu'a, near Baghdad, continues to stir curiosity among the public and controversy among the experts.
At first sight the pottery jar was not all that unusual; it was long and cylindrical rather than bowl-shaped, but jars with such a shape are not unknown from the period at the beginning of the Christian era. Only when the archaeologists began to remove the dirt that filled the jar was the unusual part discovered. The interior of the jar was lined with copper and this inner copper cylinder was, in turn, filled with bitumen. Firmly fixed into the bitumen was an iron rod.
Various suggestions were advanced as to the purpose of this curious contrivance. Archaeologists have a tendency, when confronted with the unusual, to describe it as a "ritual object" and no doubt this would have satisfied most. Indeed, as recently as 1989 a certain E. Paszthory published a paper in MASCA Research Papers in Science and Archaeology entitled, "Electricity Generation or Magic? The analysis of an unusual group of finds from Mesopotamia" in which he outlines the magical meanings of metals and argues that the objects were containers for incantations written on papyrus.
The controversy really began, however, when someone pointed out that if the jar was filled with an acid solution such as vinegar or lemon juice, there should be a current flowing between the two metals. The thought that electricity might have been invented 2,000 years ago was more than most historians and scientists were prepared to admit and their scepticism seems justified. As some experts have pointed out, high school science experiments notwithstanding, lemons do not produce much current. Benzoquinone is a more effective electrolyte, but although quinones occur naturally in the secretions of some beetles and centipedes, the chances of someone stumbling across this fact are pretty small and the difficulty of collecting sufficient material seems formidable.
Even more to the point, the jar found by the archaeologists was, thanks to its bitumen coating and copper lining, air tight. Oxygen is consumed by the reaction that produces electricity and once the oxygen in the container was used up, current flow would sink to virtually zero.
However the controversy continues, mainly due to the fact that similar jars were found at Seleucia-on-the-Tigris and at Ctesiphon. The Seleucia jars had bronze cylinders inside which were sheets of papyrus while the Ctesiphon jars contained rolled bronze sheets. Both these jars were porous, which would allow oxygen to enter.
Back in the 1930s a certain gentleman by the name of Konig studied the techniques used by jewellers in Baghdad and observed, rather to his astonishment, that the silversmiths were using primitive batteries for gold plating. Electro-plating was supposedly developed by John Wright, a Birmingham inventor, in 1839 and Konig assumed that the Baghdad artisans had somehow heard of Wright's work and copied it.
We may, however, wonder whether that assumption is correct. Is it possible that the Baghdad jewellers were working according to a tradition that is at least 2,000 years old and that the curious objects were indeed batteries? Possibly we shall never know, but we can at least speculate.